1. Field of the Invention
The present invention relates to systems for the electrical transmission between two units mobile relative to each other. These systems are employed, for example, as linear sliding paths in units designed for linear movement such as crane installations or as slip rings for rotary transmission, e.g. in computer tomographs.
2. Description of Related Art
For slip rings or sliding paths, both contacting and non-contacting transmission techniques are known. As the engineering principle of their function for rotary transmission and the functional principle for linear or curved transmission are one and the same, a more detailed distinction will not be made between these techniques in the following. The terms will be used synonymously. The operation of a contacting transmission will be briefly explained below with reference to the example of a slip ring. Circular or annular sliding paths are mounted on a first unit. A second unit, with a sliding contact resting on the sliding path and in sliding contact with the latter during the movement, is moving relative to this first unit. Now an electric current can be transmitted via this galvanic contact.
It is equally possible to move the sliding contact at a small distance above the sliding path so that there is an exclusively capacitive coupling between the sliding path and the sliding contact. High-frequency signals or high-frequency signal fractions can be transmitted via this capacitive coupling without any problems.
Various technologies have become known for the mechanical structure of such slip rings. For example, solid sliding paths turned from cylindrical metal bodies can be stacked together with insulating bodies to form a slip ring with several transmission paths. Such slip rings excel themselves by a long service life. However, the manufacturing costs are comparatively high.
Slip rings realized on PC boards in the form of printed circuits are mostly more expedient to manufacture. One example of such a slip ring is disclosed in the German Patent Application DE 196 01 965 A1. Here, the sliding path is a circular conducting path on the PC board. However, in that case, a number of specific additional steps of operation are required in manufacture, compared against the conventional PCB technology. For instance, the conducting path is provided with a surface coating consisting of silver or gold. Such slip rings can be manufactured at lower costs than the previously described solid slip rings but their service life is shorter. A slip ring manufactured with application of the PCB technique is disclosed in the German Patent Application DE 196 01 965 A1, for example.
Slip rings are easier to manufacture in the so-called stacking technique that is disclosed, for instance, in the German Utility Model DE 298 00 281 U1. These rings consist of strata of electrically conductive material and insulating material, which are stacked in alternation.
Such slip ring technologies are well suitable for the transmission of low-frequency direct current or alternating current. For higher frequencies, a defined natural impedance and a defined shielding are required. The defined natural impedance is necessary in order to permit a reflection-free transmission. When the natural impedance varies along the slip ring or when it does not correspond to the natural impedance of the connected lines, reflections may occur that result in interference with or distortion of the transmitted signal.
Moreover, particularly in the case of higher frequencies, a defined shielding is desirable in order to comply, on the one hand, with the applicable EMO standards and, on the other hand, to reduce the crosstalk with neighboring lines. Hence, cross coupling or crosstalk between neighboring slip ring paths is one of the most annoying problems in the transmission of high-frequency signals. For the reduction of crosstalk, a solution based on balancing transformers, for example, is known from the German Patent Application DE 19627628 A1. That solution entails the advantage, however, that the respective balancing transformers must be matched with the respective conditions in terms of impedance and frequency. Moreover, the voltage-proof characteristics of the transformer reduce the voltage-proof characteristics of the system as a whole. This transformer limits, as a rule, also the ampacity.
The U.S. Pat. No. 5,530,425 discloses another approach. There, the sliding paths are accommodated in a metal-plated trough. This solution is very expensive and consumes much space. For example, in production in a solid plastic support it is necessary to turn the trough out and to provide it with an adhesive metal film or a galvanic metallized layer bearing a conductive coating on the inside.